What is a very good bitrate for 4K video files above which it does not make sense to go?

That is too broad of a question for a too narrow of an answer. You can answer with broad statements and generalized estimations, but I don't think they really answer the question.

Encoding video balances three things (extensible by two more):

• visual quality / equivalence
• size (stream/file size)
• encoding time
• decoding performance
• decoding feature set (compatibility)

The codec you use also has a high impact on compression ratio opportunities and capabilities. AV1, HEVC, AVC? 10-bit?

If we define that we do not care about encoding time, so we will use the very slow preset and use all codec features available, compression ratio and quality falloff still depends a lot on what you actually encode.

• Is it a cartoon with flat surfaces and mostly linear and partial linear or transformative movement? That can be compressed very well through differentials and transformation (movement).
• Is it a high-grain cartoon or movie? Fine, noisy details are hard to compress, they require more information.
• Does it have a lot of movement? A lot of vast movements and cuts? Less to keep and differentiate data with, so less efficient.

I suspect in higher resolutions the gaps between different visual data compression ratio differs more - because a difference is elevated through higher resolution/more data.

That being said, I don't have or use 4K stuff, so I can't even check for some rough numbers and visual content to size differences.

There is no "this much is very good enough for 4K movies" because the reasonable minimum very much depends on what the movie contains.

One consideration for me is: how grainy is the source material, and how much do I care about retaining that? Because film grain is the first thing to go when you apply too much compression. Dark scenes, too.

For kids movies or something I’ll watch casually: 15gb x265 rips are fine.

For new releases that I want to watch and maybe will a few more times: I’ll grab the 20-30gb web-dl and enjoy that.

For a movie I consider a masterpiece and want the best possible? Give me the 50-80gb remux.

At the end of the day it's about what you like, what is available, and how much space you got.

My rule of thumb has been 8GB per hour of content for 4K (I don't remember where I heard of got this from, so at the end of the day, this is just some arbitrary number). I usually stick to x265 encodes and so far this had been good enough for me. Some prefer the best (untouched remux), but like you mention, these files are huge. Even though I have many drives, I dont want each movie being 70+GB per file. Sometimes I break my rule of thumb and do get "higher quality" (that isn't a remux). I think the biggest file I have is around 50GB for a x265 2160p encode of a movie where a certain king returns. A with everthing, there are exceptions. Just do what you want.

This has been good enough for me. Obviously, the bitrate of audio matters and its format. That, in and of itself, is a whole other issue ( lossless or not, channels, etc..)